1. Technical Field of the Invention
The present invention relates to cellular telephone systems and, in particular, to a method and apparatus for enhancing call set-up and handoff quality by accounting for the differing broadcast coverage characteristics of the plural antennas used by cellular telephone system base stations.
2. Description of Related Art
Reference is now made to FIG. 1 wherein there is shown a schematic view of a cellular telephone system 10 including a plurality of individual cells 12. Each cell 12 operates with a different assigned set of transmission frequencies f selected from the cellular frequency band. The set of frequencies assigned to each cell 12 includes at least one control frequency f.sub.c operable in either or both an analog and/or a digital mode, and a plurality of voice frequencies f.sub.v also operable in either or both an analog and/or a digital mode. Sets of assigned frequencies are usually (but not necessarily) different for adjacent cells 12, and such sets are not repeated except for cells that are far enough away from each other to minimize the likelihood of adjacent or co-channel interference.
A base station 14 including a plurality of transceivers (not shown) capable of operating independently on each of the assigned set of frequencies is provided for each of the cells (one set of frequencies per base station). Via the transceivers, the base stations 14 engage in simultaneous communications with plural mobile stations 16 operating within the area of the associated cell 12. The control frequencies f.sub.c in the set of frequencies assigned to each cell 12 are used to carry system control signals between the base station 14 and proximately located mobile stations 16. Such control signals include page signals, page response signals, location registration signals, voice channel assignments, maintenance instructions, and cell selection or reselection instructions. The plurality of voice frequencies f.sub.v in the set of frequencies assigned to each cell 12 are used to carry subscriber voice or data communications between the base station 14 and proximately located mobile stations 16.
The base stations 14 further communicate via data links (and voice trunks) 22 with a central control station, commonly referred to as a mobile switching center 18, which functions to control operation of the system 10. In particular, the mobile switching center 18 operates to selectively connect subscriber voice and data communications to the mobile stations 16 through the base stations 14. Thus, the mobile switching center 18 controls system operation through and in response to the transmission of control signals over the control frequencies f.sub.c to set-up on the voice frequencies f.sub.v calls that are either originated by or terminated at the mobile stations 16. The mobile switching center 18 further controls, through and in response to control (pilot) frequency f.sub.c transmissions, and measurements on voice frequencies f.sub.v the handoff of a subscriber communication from a voice frequency f.sub.v of one cell 12 to a voice frequency f.sub.v of another cell as the subscriber mobile station 16 roams throughout the cellular service area during an ongoing communication.
At call set-up, for example when a subscriber originates a call from a mobile station 16 or the system 10 receives a call to be terminated at the subscriber, the mobile station has already selected a cell 12 and base station 14 through which a call originated by or terminated at the mobile station is to be transmitted. What has not yet been selected, however, is the particular one of the voice frequencies f.sub.v to carry the call. This frequency selection is made from the currently unused voice frequencies f.sub.v within the set of voice frequencies f.sub.v assigned to that cell 12 which was selected by the mobile station 16.
During an ongoing call, the communication is already being carried by one selected voice frequency f.sub.v. As the mobile station 16 roams around the broadcast coverage area of a currently serving cell 12, the mobile station makes signal strength measurements on a measurement channel (constantly transmitting pilot frequency) for each of its neighboring cells. When the mobile station roams out of the currently serving cell, the signal strength measurement on the measurement channel of one of the neighboring cells exceeds the received signal strength on currently used voice frequency f.sub.v. At that point, the mobile station 16 makes a request of the cellular system for a handoff. The mobile station made measurements are reported back to the mobile switching center 18, and a new cell 12 and base station 14 are selected based on the measured signal strengths to handle the call. A voice frequency f.sub.v assigned to the new cell 12 is then selected, and a command is transmitted from the mobile switching center 18 ordering the mobile station 16 to handoff to the selected voice frequency in the new cell. At the same time, the mobile switching center re-routes the call from the base station 14 in the old cell 12 to the base station for the new cell.
Each base station 14 utilizes a plurality of antennas 20 for transmitting and receiving over the set of frequencies f assigned to the cell 12. In one implementation, different antennas 20 are used for transmitting and receiving. In fact, plural transmitting antennas 20t and plural receiving antennas 20r may be used in situations where poor antenna configuration exists or where are combiner limitations. For example, it is known to have the voice frequencies f.sub.v assigned to a cell 12 include a group of frequencies in a normal frequency band and a group of frequencies in an extended frequency band. In such a case, the voice frequencies f.sub.v in the normal frequency band may be transmitted by one antenna 20t.sub.1 of the base station 14 while the voice frequencies f.sub.v in the extended frequency band may be transmitted by a second antenna 20t.sub.2. The reference frequencies f.sub.r (comprising for example the control frequency f.sub.c, pilot frequency and/or measurement channel) for the cell may be transmitted on one of the transmitting antennas 20t along with the voice frequencies, or may instead use their own transmitting antenna 20t.sub.3. Similar arrangements (although not shown) may be made with respect to the receiving antennas 20r.
In general, the cellular telephone system operates well during either call set-up or handoff to select a voice frequency f.sub.v. However, the use of plural antennas 20 for transmitting and receiving over the set of frequencies f assigned to the cell 12 introduces a potential problem in the selection of a voice frequency f.sub.v during either call set-up or handoff. This problem arises when the broadcast coverage characteristics of the antenna 20 used to transmit/receive the reference frequencies f.sub.r differ from the characteristics of the antenna 20 used to transmit/receive one or more of the voice frequencies f.sub.v. When different antennas 20 are used, the expected signal strength of the selected voice frequency f.sub.v for the call may dramatically differ from the measured signal strength on the reference frequency f.sub.r. A call then set-up on, or handed off to, that selected voice frequency f.sub.v by the mobile switching center 18 may not be satisfactory.
The foregoing problem may be better understood with reference to FIG. 2 wherein there is shown a schematic diagram illustrating an example of the differing broadcast coverage characteristics 26 commonly experienced with the use of plural antennas 20 by cellular telephone system base stations 14. Suppose the cell 12 in which base station 14 operates is assigned use of a set of frequencies f from the cellular frequency band that include the reference frequency f.sub.r (comprising a control frequency f.sub.r, pilot frequency and/or measurement channel) and a plurality of voice frequencies f.sub.v in both the normal and the extended bands. Further suppose that the base station 14 includes a plurality of antennas 20 for transmitting and receiving communications over the assigned set of frequencies. A first one of those antennas 20 comprises a transmitting antenna 20t.sub.1 assigned to transmit on the reference frequency f.sub.r and those voice frequencies f.sub.v in the normal band. A second one of those antennas 20 also comprises a transmitting antenna 20t.sub.2, but is assigned to transmit on those voice frequencies f.sub.v in the extended band only. Similar assignments are made with respect to receiving antennas (not shown to simplify the illustration) of the base station 14. Other arrangements could lead to a similar problem, for example, if the reference frequency f.sub.r is an extended voice frequency f.sub.v.
The broadcast coverage characteristics 26(1) and 26(2) for the two transmitting antennas 20t.sub.1 and 20t.sub.2, respectively, differ with respect to each other within the coverage area of the cell 12. These differences in the broadcast coverage characteristics 26 for the antennas 20t may be the result of, for example, signal fading, obstruction and reflection, or perhaps other sources of interference known to those skilled in the art. For a mobile station located at point A in the cell 12 and involved in a call on a neighbor of this cell is the differences in broadcast coverage characteristics 26 for the antennas 20t is of little concern as the received signal strength at point A from either transmitting antenna, 20t.sub.1 or 20t.sub.2, is substantially the same. At point B within cell 12, however, the received signal strength substantially differs between the two transmitting antennas 20t.sub.1 and 20t.sub.2. A satisfactory signal strength measurement at point B on the reference frequency f.sub.r broadcast from antenna 20t.sub.1 is not necessarily indicative that a satisfactory received signal strength may be expected with respect to the voice frequencies f.sub.v in the extended band broadcast from antenna 20t.sub.2. Thus, mobile switching center selection of one of the voice frequencies f.sub.v in the extended band at call set-up or handoff may fail or be unsatisfactory with respect to the mobile station at point B even through the received signal strength on the reference frequency f.sub.r and the voice frequencies f.sub.v in the normal band is otherwise satisfactory.
There is accordingly a need then for a method and apparatus for use in a cellular telephone system at voice frequency f.sub.v selection during call set-up or handoff that would account for the differing broadcast coverage characteristics 26 of the plural antennas 20 used by cellular telephone system 10 base stations 14.